1. Field of the Invention
The present invention relates to a multiplexed matrix display screen and to its control process. This screen makes it possible to display in black and white or colour, with or without half-tones, simple or complex images or pictures making it possible in particular to display moving pictures of the television picture type. The invention also applies to screens using an electroluminescent material or microdot cathodoluminescent screens.
2. Brief Description of the Related Prior Art
It is known that for the control of the display of images or pictures on a matrix screen, to each row and to each column of the screen is allocated an electrode and a control circuit and the screen is addressed one row at a time. For n rows, the multiplexing is of order n and the total image time T is subdivided into row time intervals T/n=T1. each of these intervals being allocated to the writing of image points or pixels of a screen row.
For the duration of a row period or time T1, the row addressed or selected by the short row scan is raised to a so-called selection potential Vls. During this time, the m columns are raised to potentials appropriate for the display of informations on the pixels of said row.
In the case of a solely black and white display, also referred to as digital display, either the potential Vc or -Vc is applied to the columns as a function of whether it is wished to respectively display black or white.
The unaddressed or unselected rows are, as a function of the screen type, either raised to a non-selection potential Vlns, or are left floating or placed under high impedance.
The invention is well suited to cathodoluminescent screens using emitting microtips of electrons supported by column electrodes which then serve as cathodes, the row electrodes resting on the column electrodes being isolated from the latter and perforated facing the microtips and then function as grids. One or more cathodoluminescent anodes are positioned facing the microtips. Generally a cathodoluminescent anode is constituted by an anodic conductor covered with a luminescent material under electron bombardment.
The article by T. Leroux et al "Microtips display addressing", SID 91, P. 437 contains a description of the operating principle of microtips cathodoluminescent screens and the manner of addressing them. In these known microtips screens, the unselected rows are raised to an imposed non-selection potential.
It is stated in the above article that one of the major disadvantages of such a screen is the electric power consumed during digital column addressing. Thus, the structure of a microtips screen leads to the appearance of a high row-column capacitance at each row selection and this can be discharged or charged at the column control voltage Vc.
The consumed capacitive power is then P-1/2CVc.sup.2 Fm, in which P is the consumption per dm.sup.2, C is the capacitance per dm.sup.2, Vc is the column modulating voltage and Fm the effective modulating frequency of the column signals.
In the particular case of the display of a uniformly grey background obtained by a time modulation method, the frequency Fm is equal to twice the scanning frequency Fl of the rows and the capacitive consumption is then at a maximum. In practice, for a capacitance of 30 pF/mm.sup.2, a voltage modulated on 30 V columns and a row scanning frequency Fl or 30 kHz leads to a consumption of 8 W/dm.sup.2.
ACTFEL electroluminescent screens use a fine electroluminescent material layer place between the row electrodes and the column electrodes. This type of screen is more particularly described in the article "Display Drive Handbook" 1984, Texas Instruments, "The AC Thin Film Electroluminescent Display", pp 2-43 to 2-49.
According to this article, the addressing sequence of each row is as follows:
1) During the selection time of one row, the potential of the row addressed is firstly raised to a potential V.sub.neg, the potentials of the columns being raised, as a function of the information to be displayed, to +Vc or 0.
2) The selection time has a second phase during which the potentials of the selected row and all the columns are reduced to 0.
3) One then passes onto the addressing of the following row.
The previously selected row passes into a high impedance state HZ and the row potential is then floating.
This "floating row" principle is already widely used for the addressing of electroluminescent screens, which have electric power consumption problems similar to those of cathodoluminescent screens (cf. J. P. Budin, "Principes d'adressage des ecrans matriciels" General Display Education Seminars--Visu 90).
The direct transposition of this control mode to microtips screens could be possible and interesting from the capacitive consumption standpoint. However, compared with the control mode generally used in cathodoluminescent screens (imposed row non-selection potential), it would lead to a significant luminance loss. Thus, the times during which the rows would be brought to zero would be taken on the addressing time of the selected row. However, in the particular case of microtips screens, the luminescence is directly proportional to the addressing time.
As users are requiring ever more complex screens it is necessary to be able to bring about an optimum use of the addressing times and therefore eliminate dead times.
For microtips screens, a discharge time is necessary after each row selection. Thus, in this type of screen, any voltage exceeding the threshold voltage immediately leads to the emission of electrons at the tips and therefore light at the front face (cathodoluminescence phenomenon.) However, the selection of a row takes place by raising the latter to a potential close to the threshold, the column potential translating the information to be displayed.
Thus, having imposed this potential on the first or row terminal of the row-column capacitance, if it was merely adequate to "open the switch", the potential of said row would not instantaneously vary in order to make the row unselected, but would instead very slowly return to the mean potential of the columns. Consequently by adding the column potentials intended for the following rows, there would be a succession of parasitic emissions. Multiplexing is not possible under these conditions. It is therefore absolutely necessary to discharge the row immediately after its selection time. The discharge time corresponds to the time necessary for the outflow of all the charges stored in the previously addressed row.
A single pulse having a virtually negligible duration compared with the selection time cannot be used for solving this problem. Thus, it is not sufficient to raise the row electrode to the desired potential, it also being necessary to eliminate all the charges stored in the "reservoir" constituted by the distributed charge row, formed by the row electrode (of non-zero resistivity) coupled to its system of row-column capacitances, the second terminal of said capacitances being respectively connected to a high resistance (resistive layer located between the microdots and the corresponding column electrode). In practice, the time necessary for the dissipation of the charges exceeds approximately 10 microseconds.